Technical Field
This disclosure relates to magnetic resonance imaging (MRI).
Description of Related Art
Spatial aliasing may occur in an magnetic resonance (MR) image if the k-space sample spacing is not fine enough to support the full spatial extent of the received signal. This sampling criterion may become burdensome and inefficient when the region of interest (ROI) is small relative to the full spatial extent of the anatomy. With reduced or restricted field-of-view (rFOV) imaging, efficiency may be improved. In an rFOV acquisition, the prescribed FOV may be decreased to more closely match the ROI size, thereby permitting shorter readout durations (Tread), fewer k-space segments, or both. The time savings can be leveraged for faster acquisitions, improved tolerance to artifacts from motion or off resonance, finer spatial resolution, and/or better coverage.
rFOV techniques may employ outer volume suppression (OVS), a spatial pre-saturation sequence which attenuates signal outside the ROI and serves as a spatial anti-aliasing filter. Energy along the readout direction may be suppressed by hardware filters, so typically OVS may be applied to reduce signal along the direction(s) orthogonal to the readout. Compared to multi-dimensional excitation pulses, whose long durations can negate the time-efficiency benefit of an rFOV acquisition, Stenger A V, Boada F E, Noll D C Multishot 3D slice-select tailored RF pulses for MRI, Magn Reson Med 2002, No. 48:157-168; Deng W, Stenger V A, A three-dimensional variable-density spiral spatial-spectral RF pulse with rotated gradients, Magn Reson Med 2010, No. 63:828-834, OVS preparations may be applied prior to a set of readout intervals (TR) and may provide more design flexibility.
OVS sequences may be effective only at lower field strengths (≦1.5 T) due to their sensitivity to B1+ inhomogeneity, or have long durations that may preclude their utilization in cardiac and other rapid imaging applications. Dynamic radial acquisitions with REST, which uses cosine-modulated saturation pulses to reduce the FOV orthogonal to each spoke, has been applied in 1.5 T cardiac imaging. However, this method may not be not suitable for spiral imaging, may need to be applied once each TR interval, and may be B1+ sensitive.
A faster, single-sided OVS design, which ca be used in rapid single-shot spin-echo cardiac imaging, may also not be robust to B1+ variation. The BISTRO method may be capable of 2D OVS with improved B1+ field insensitivity, but its duration may be over 200 msec. A 1D OVS design may be used for spinal cord diffusion imaging at 3 T. However, the duration may be almost 100 msec due to the need for multiple quadratic-phase pulses.
Pisani, et al. used a rapid OVS sequence for functional MRI of the fetal brain at 1.5 T. The OVS design consists of a long 2D spatially selective spiral tipdown pulse, followed by a B1+-insensitive adiabatic half-passage tipback pulse and spoiler gradients. The sequence may suppress signal outside of a cylindrical ROI, and may be suited for multi-slice spiral or radial imaging.